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A vast amount of technical testing was undertaken to determine the effectiveness of Sigma's optical stabilisation system after it became apparent that it seems more likely to degrade the recorded image quality than to improve it My tests show that using the Sigma's Optical Stabiliser in Mode 1 lowers and flattens the MTF figures, with the least effect at the minimum focal length and the greatest effect at the maximum focal length. Mode 2 has almost no effect at the minimum focal length but degraded the MTF figures above f/11 for300mm and above f/8 at 500mm.

Correlating the focal lengths with the prevailing exposure times for Mode 2's more adaptive algorithm it became apparent that the OS effect cuts-in roughly when the exposure time exceeds the reciprocal of the focal length. But the OS system's effect appears to be more derogatory than the blur it attempts to correct!

In short and as Sigma suggests on its website, blur-reduction is a method for improving usability, by making composition and

manual-focusing easier, rather than a technology to enhance image quality.

There is, of course, the option to increase the ISO in order to use a shorter exposure time to combat camera shake. Separate MTF tests to assess the effect of image noise showed that switching from ISO 100 to 1600 pulled the Tamron's performance down slightly but the effect was less than the degradation seen when the Sigma's OS was activated. Tamron's lens is dearly the better of the two. Sigma's will be preferred by users who prioritise ease of use over image quality. WDC